Variable focal length ophthalmic lenses are a great value particularly in the area of eyeglasses or spectacles. The human eye, after a person reaches the mid forties, looses its natural flexibility for focusing on both near and far objects. As a consequence, most people require spectacles to assist them in both reading and long-distance vision. Several products are available for this purpose: reading spectacles, bi-focals spectacles, and multi-focal or progressively graded spectacles; and spectacles with a variable power lenses. Each of these approaches has its own drawbacks. Reading spectacles must be put on before beginning to read, assuming they can be found when needed. Bi-focals allow closeness in viewing but only at restricted low angles which generally require the wearer to tilt his head upwardly to an unnatural position. Progressively graded spectacles allow correction for a continuum of distances, and have the advantage of permitting the user to instantaneously change focus in accordance with a change in viewing angle. With progressively graded lenses, focal distance strictly depends on the direction of viewing which is not necessarily commensurate with environment requirements. The largest power grading starts from the highest power levels at low angles for reading, and becomes gradually lower for higher angles and thus longer distances. Thus, the lens power usually is graded to decrease by up to three diopters from bottom to top angles of viewing. The problem with angle-dependent, progressively graded lenses is best illustrated in the case of near objects positioned at high viewing angles. This dictates an extremely unnatural tilting of the head in order for the user to see the objects well.
In the normal design of progressive graded lenses, the power variation provides for a variation in power for objects at about 30-40 cm. at low viewing angles, to infinity at the most elevated angle. In general, the extent of such a power variation is about 2.5-3 diopters from the lowest to the highest viewing angle. This variation of power as a function of viewing angle, as dictated by any progressive grading technique, is very restrictive because of the objects in an environment viewed by an observer rarely exhibit a spatial positioning that is complementary to the grading. In most scenarios, the line of sight of a near object is lower than that of more distant objects, but this is not always the case. Moreover, even if distribution of distance to objects is monotonic with angle, this distribution function, in a real environment, is likely to exhibit an angle dependency different from the power correction that a given lens may provide for. The best that can be hoped for is a reasonable statistical matching of the rate of change of lens power versus distance distributions of objects in a typical scene.
It is thus obvious for designs of this type, that rapid refocusing from near to far objects will frequently require an inconvenient tilting of the head of the wearer. Such tilting is essential for the eyes to gain a viewing angle for which the lens provide the right power or focal distance.
Spectacles with variable power lenses in which the focal length can be changed in response to the touch of a switch or moving a lever on or near the frame of the glasses, are obviously of potential advantage to a user. U.S. Pat. Nos. 4,261,655 and 4,418,990, and copending application Ser. No. 208,660, now U.S. Pat. No. 4,913,536, disclose fluid-filled flexible lenses which can be adjusted to provide variable power. The flexibility of the materials and the fluid in these lenses permit controlled variation of their focal lengths.
A potential limitation of the spectacles employing variable power lenses is the time required to change the power by the user, as well as the inconvenience for having to effect operation of the apparatus by which the power is changed. Although sub-second switching times are possible, all too frequently changes in the lens power are unacceptable because of user fatigue, or excessive energy drain if the mechanism for effecting changes is automatic.
For glasses with a variable power lens, the power is controllable at any point on the lens. Thus, any object in sight can be brought into focus without the need to tilt the head. However, there are many situations which requires frequent changes in the focus, and the process may become inconvenient due to need for repetitive control activation.
The problems faced by many persons over forty as illustrated in FIG. 1 which shows typical distances and viewing angles for an operator of a video display terminal. In normal use, an operator must continuously shift his line of sight through an angle by about 60.degree., the distance to an object varying from about 0.4 m. when viewing a keyboard to about 0.55 m. when viewing the screen of the terminal. Maximum variation in power, DP.sub.max under these conditions is 0.7 diopters where DP.sub.max =1/D.sub.s -1/D.sub.l, and D.sub.s is the shortest distance to an object and Dl is the longest. Thus, the eye of an operator under these conditions must be able to accommodate a range of 0.7 diopters.
For ordinary desk work DP.sub.max is about 1.25 diopters as shown in FIG. 2 wherein the eye is about 0.4 m from a desk surface, and an angular range of about 60.degree. is typical. Progressively graded lenses may be utilized as shown FIG. 3A. Typically, the variation in power is as indicated in FIG. 3B. Variation in power along the vertical axis of the lens ranges from 0 at a 30.degree. upward angle measured from the reference indicated, to about 1.25 diopters at about a 30.degree. downward angle from the reference and aligned with a keyboard, for example. To properly see objects on the floor when a person is standing, DP.sub.max is about 0.34 diopters as shown in FIGS. 4A and 4B.
From the above discussion one can see that a wide variation in power is required for a person to properly see a computer keyboard on a desk, the screen of a video terminal, and to see across a room.
An object of the present invention is to provide an improved ophthalmic lens which minimizes the drawbacks described above.